1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser diode used as a light source for optical data processing or high-speed optical communication and a manufacturing method of the laser diode.
2. Description of the Related Art
In the technical field of the optical communication or optical recording, there has been a lot of interest in a surface emitting semiconductor laser diode (Vertical-Cavity Surface-Emitting laser diode, which will hereinafter be abbreviated as VCSEL) in recent years.
The VCSEL is characterized in that its power consumption is low owing to a low threshold current, a circular optical spot is easily available, and evaluation in the stage of wafer and two dimensional array of a light source can be achieved. These are excellent characteristics which the conventional edge emitting laser diodes do not have. On the other hand, the VCSEL cannot readily yield a high optical output exceeding 10 mW when it is used solely because “the volume of its active region is small” which is also a cause for the low threshold current. In addition, it has a device resistance typically from dozens to hundreds of Ohms, which is remarkably higher than that of the edge emitting laser diode (several Ohms).
The optical communication via an optical fiber has been employed for medium or long distance (from several to dozens of kilometers) data transmission. A combination of a single mode optical fiber made of silica and a laser having an oscillation peak in a long wavelength region of 1.31 μm or 1.55 μm has been used for it. They are light sources having excellent characteristics such as “low dispersion in the fiber or considerably small transmission loss”, but involve many problems. For example, they need the temperature control of the device and take time for alignment of optical axes between the optical fiber and laser. Since telecommunication carriers are main users and therefore, the production is carried out on a small scale compared with that for the consumer product, the system itself is inevitably expensive.
In recent years, the widespread of asymmetric digital subscriber line (ADSL) and cable television (CATV) at home has made it possible to transmit data having ten times to hundred times the capacity of the conventional data. Use of the Internet is expected to increase further in future. There will be an increasing demand for high-speed and high-capacity data transmission and undoubtedly, the optical fiber will be commonly used at home in some day.
It is, however, uneconomical to employ a combination of a single mode type optical fiber and a distributed feedback (DFB) laser, which has been used for the medium- or long-distance communication, for example, for a few meters to dozens of meters data transmission between home and a telephone pole. For such short-distance communication (several to hundreds of meter), a low cost optical fiber such as a multi-mode type silica fiber or a plastic optical fiber (POF) can be used economically. The light source to be used for such a multi-mode type optical fiber therefore must satisfy the following requirements: the device itself is inexpensive and in addition, it does not need a special optical system or a driving system. A surface emitting laser is therefore one leading option capable of satisfying these requirements.
In the local area network (LAN) which is an indoor network, its transmission speed of data starting at ten megabits per second (Mbps) has increased to hundreds of megabits per second. Recently, the data transmission speed of 1 gigabit per second has appeared. It will really reach ten gigabits per second in near future. An electrical interconnection using a twist pair cable can be applied to the transmission speed up to 1 gigabit per second. At a data transmission speed exceeding 1 gigabit per second, however, it will be replaced with optical interconnection in consideration of the limit of noise tolerance.
There is an active movement to adopt the VCSEL for a light source of the optical interconnection used in the Ethernet (trademark) of 10 gigabits per second and its development has already begun. As explained above, no problem exists in the modulation at several GHz, but some measures must be taken to heighten the response characteristics to 10 GHz or greater.
A theoretical review of the modulation band of a laser diode is described in detail, for example, in “Semiconductor Laser”, ed. by Kenichi Iga, Ohmsha, 1990”. A relaxation oscillation frequency (fr) which provides an indication of an improvement of the modulation band is derived by the following equation (1) based on the rate equation.
                              f          r                =                              1                          2              ⁢              π                                ⁢                                                    ξ                ⁢                                                                  ⁢                                  G                  ′                                ⁢                                  P                  out                                                                              η                  d                                ⁢                h                ⁢                                                                  ⁢                                  ω                  o                                ⁢                                  V                  m                                                                                        (        1        )            
In the equation, ξ is an optical confinement coefficient, G′ is a derivation gain coefficient, Pout is an optical output, ηd is an external derivation quantum efficiency, and Vm is a mode volume of a resonator. Described specifically, based on the finding that fr increases in proportion to a square root of the optical output, the optical output is often heightened by increasing an injected current (bias current) in order to improve response characteristics.
It is, however, known that an increase in the injected current leads to an increase in the current density (current per unit area) in a light emitting region, which adversely affects the life characteristics of the device. For preventing such an influence, there may be a method of widening the diameter of a current confining portion to lower the effective current density. As can be understood from the above-described equation, however, the method increases a mode volume Vm of a resonator and does not contribute to a substantial improvement in the response characteristics.
From another standpoint, there is a danger of the optical output exceeding the safe standards called “Eye Safe” owing to its increase. According to JIS C 6802 in which safety standards of laser products are specified, a laser with a wavelength of 850 nm having an optical output of 0.78 mW or less belongs to Class I (safety level: exposure to 30000 seconds (=8 hours) does not cause any damage to eyes). If the laser may exceeds this safety level, an additional device such as monitor device for monitoring or limiting the optical output or a shielding plate for preventing optical leakage must be incorporated in a system or module.
Here, fr can be represented by another equation:
                              f          r                =                              1                          2              ⁢              π                                ⁢                                                    ξ                ⁢                                                                  ⁢                                  G                  ′                                ⁢                                  S                  0                                                            τ                p                                                                        (        2        )                                          S          0                =                                            τ              p                        ⁡                          (                                                I                  0                                -                                  I                  th                                            )                                            e            ⁢                                                  ⁢                          V              a                                                          (        3        )            
In the equation, G′ is a derivation gain coefficient, τp is a photon lifetime, ξ is an optical confinement coefficient, and S0 is a stationary solution of a photon density at an injected current I0. This equation suggests that a substantial improvement in response characteristics can be brought about by lowering of a threshold current Ith even when the injected current is equal.
For improving the response characteristics, a method of increasing fr while narrowing the diameter of a light emitting region (decreasing the volume of an active region), reducing a threshold current and reducing an injected current is presumed to be effective.
The 3dB down cutoff frequency (f3dB) which is an indication of a modulation band can be represented by the following equation when the inductive reactance is negligibly small.
                              f                      3            ⁢            d            ⁢                                                  ⁢            B                          =                  1                      2            ⁢            π            ⁢                                                  ⁢            CR                                              (        4        )            
This means that the modulation band varies depending on a CR time constant. Narrowing of the diameter of a light emitting region leads to an increase in the element resistance and a reduction in cutoff frequency caused thereby offsets an improvement in the response characteristics. An increase in the element resistance is also undesirable from the viewpoint of driver impedance matching.
With the foregoing problems in view, the present inventors have proceeded with an investigation on a method of improving response characteristics while suppressing an increase in an injected current as much as possible. As a result, the investigation has reached a conclusion that a reduction in a device capacitance and also in a threshold current while maintaining an element resistance without changing the diameter of a light emitting region is effective. It has been found that this can be attained by such a structure that a static capacitance reducing portion having a capacitance component smaller than a junction capacitance is added in series with a pn junction and at the same time, the reflectance of a resonator is heightened as much as possible until it permits this static capacitance reducing portion to play a partial role of a reflective film and extract an optical output.
Structures aimed at reducing the static capacitance of a surface emitting semiconductor laser diode have so far been proposed. Typical structures are a multi-oxide structure and a hybrid confining structure utilizing selective oxidation and proton injection.
In the multi-oxide structure as disclosed in Japanese Patent No. 3116088 and Japanese Published Unexamined Patent Application No. 2003-168845, plural layers of an oxidized portion are formed in a post by inserting plural layers different in an oxidation rate under the same temperature environment. In this method, plural layers different in an aluminum composition ratio are inserted in advance upon crystal growth or plural layers equal in an aluminum composition ratio but different in the film thickness are inserted. A static capacitance is reduced by substantially thickening the film of an insulating region, while controlling the light cladding intensity by increasing an oxidation depth of the oxidized portion only on a side near the active layer compared with that on the other oxidized portion.
In Chih-Hao Chang, “Parasitics and design considerations on oxide-implant VCSELs”, IEEE Photon. Technol. Lett., 13, 1274 (2001), disclosed is a laser diode having a hybrid structure in which selective oxidation and ion injection are used in combination. Similar to the above-described multi-oxide structure, this structure also aims at a reduction in the device capacitance by thickening an insulating region.
In the reduction in static capacitance according to the method as described in Japanese Patent No. 3116088 and Japanese Published Unexamined Patent Application No. 2003-168845, it is important to control an aluminum composition ratio in a material used for a semiconductor multilayer film or a thickness of the film with high accuracy to cause a difference in the film depth, so advanced technology is necessary for crystal growth. If there is a difference in the aluminum composition ratio or film thickness between what is formed and what is desired, it has a direct influence on the oxidation depth. Within a plane of a gallium arsenide wafer having a diameter of 2 or 3 inches, such distribution results in fluctuations in the device properties. The above-described method is therefore not suited for mass production from the viewpoint of the yield of the device, when in-plane uniformity is taken into consideration.
In the method as disclosed in Chih-Hao Chang, Parasitics and design considerations on oxide-implant VCSELs, IEEE Photon. Technol. Lett., 13, 1274 (2001), the thickness of an insulating portion formed newly by the proton injection is important. It is, however, indispensable to optimize the energy or dose upon proton injection, thereby controlling the injection depth precisely. Injection conditions must be changed, depending on the composition or thickness of a semiconductor multilayer reflective film so that it takes tremendous time to find conditions suited for practical use. This leads to an increase in the number of steps and may complicate the method.
In the structure or manufacturing method of the VCSEL, a measure permitting a reduction of a device capacitance while being excellent in mass productivity and employing only easy manufacturing steps has not yet been developed.